1. Field of the Invention
This invention relates to an apparatus, a method and a program for processing, and an apparatus and a method for reproducing, a digital signal obtained by a signal conversion process such as a digital audio signal in a form compression-coded using an irreversible compression method such as, for example, frequency correlation coding.
2. Description of the Related Art
A compression process of an audio signal is implemented by a combination of “quantization (PCM (Pulse Code Modulation) signal)”, “time correlation coding” which uses time continuity of the audio signal, “frequency correlation coding” which uses the auditory sense of the human being and “entropy coding” which uses one-sidedness of the appearance probability of codes obtained by the coding methods mentioned.
The compression techniques mentioned above are standardized by the MPEG (Moving Picture Expert Group) system, ATRAC (Adaptive Transform Acoustic Coding® system, AC-3 (Audio Code Number 3® system, VMA (Windows Media Audio® system and so forth. At present, audio signals coded by such coding systems are used over a wide range in digital broadcasts, network audio players, portable telephone systems, Web streaming and so forth.
Among the compression processes, the “frequency correlation coding” has a significant influence on the compression ratio and the sound quality. The “frequency correlation coding” orthogonally transforms a quantized PCM signal from a time domain signal into a frequency domain signal, determines deviations in signal energy in the frequency region and uses the deviations to perform coding of the PCM signal thereby to raise the coding efficiency.
Further, in the “frequency correlation coding”, the frequency band of the signal obtained by the orthogonal transform is divided into several sub bands using a psychological auditory sense and a kind of weighting is applied to the signal to quantize the signal so that signal deterioration in a frequency sub band which can be perceived comparatively readily is minimized thereby to improve the general coding quality.
In the coding which uses the psychological auditory sense characteristic, absolute audible threshold values and relative audible threshold values which depend upon a masking effect are used to determine correction audible threshold values. The correction audible threshold values are used for coding in the divisional sub bands. It is determined that those frequency components having a sound pressure lower than a lower one of the correction audible threshold values correspond to sound which may not be perceived by the human being. Such frequency components are cut or suppressed upon coding. Further, the absolute audio threshold values exhibit an increasing amplitude value in a high frequency band. Therefore, frequency components in a high frequency band are cut or suppressed more than in a low frequency band.
The compression method, for an audio signal which uses a psychological auditory sense characteristic is adopted positively by the MPEG system. The tendency of encoding of an audio signal is determined by the technical ability of encoder makers. However, in regard to an audio signal of digital broadcasting in which the MPEG system is adopted, also such a current situation is confirmed that, by the coding process described above, ail high frequency signals having frequencies higher than a certain frequency are cut or suppressed or, also within the audible frequency band, all signals in a certain divisional frequency band are cut or suppressed. Particularly where an audio signal is compressed at a low bit rate, since the number of bits which can be used for coding is small, a greater number of signals are cut by the method described above.
Several countermeasures for solving the problem of deterioration of the sound quality caused by signal deterioration by such compression coding are available as the related art. For example, Japanese Patent Laid-open No. 2002-171588, “Signal interpolation device, signal interpolation method and recording medium” (hereinafter referred to as Patent Document 1) discloses a technique regarding a method of interpolating high frequency components using an existing audio signal (interpolation object signal).
In particular, components within a first frequency band are extracted from within an interpolation object signal by means of a variable band-pass filter (BPF). Then, a logical oscillation signal from a variable frequency oscillator is mixed with the components within the first frequency band to form an interpolation signal of a second frequency band on the higher frequency band, side than the frequency band occupied by the interpolation object signal. Then, a sum signal of the interpolation signal and the interpolation object signal is outputted as an output signal.
Japanese Patent Laid-open No. 2001-356788, “Device and method for frequency interpolation and recording medium” (hereinafter referred to as Patent Document 2) discloses a technique of reconstructing a signal proximate to an original signal from a modulation wave obtained using the original signal after whose bandwidth is limited. In particular, a PCM signal is converted into a spectrum by an analyzer. Then, from among combinations of a reference band which includes the highest frequency from among frequency bands obtained by dividing the spectrum equally with the other frequency bands, that combination which exhibits the highest correlation of the spectrum distribution where one of the reference band and the other frequency band is standardized is specified by a frequency interpolation processing section.
Then, an envelope of the PCM signal is estimated by an interpolation band addition section, and a spectrum having a distribution same as the spectrum distribution in the reference band included in the specified combination is scaled by the frequency interpolation processing section so as to conform to a function of the envelope. Then, the scaled spectrum is added to the high frequency side with respect, to the reference band by the frequency interpolation processing section. Then, a signal which provides the resulting spectrum is produced by a synthesizer to reconstruct a signal proximate to the original signal.
Japanese Patent Laid-open No. 2002-073096, “Frequency interpolation system, frequency interpolation device, frequency interpolation method, and recording medium” (hereinafter referred to as Patent Document 3) discloses a method of recording information of missing signals upon coding of an original signal in advance and using, upon decoding, the recorded information to decode the original signal while maintaining the sound quality.
The techniques disclosed in Patent Documents 1, 2 and 3 are effective to solve the problem of deterioration of the sound quality.